The most commonly used sedatives and tranquilizers in horses are:
Xylazine is a common equine sedative. It can be given iv, im or (less commonly) sq and acts on alpha 2 receptors. It also provides analgesia and is synergistic with opioids for both sedation and analgesia.
Doses range from 100-200 mg iv (1-2cc of 100mg/ml) for most horses. Larger horses are more sensitive and young horses generally need more per kilogram of bodyweight. Allow horses to sedate fully before stimulating or the sedation may not take effect.
Side effects include decreased GI motility, bradycardia, nasal congestion, and decreased thermoregulation. Sedated horses put most weight on their forelimbs, making them light behind. If startled, they are more likely to kick and less likely to give warning. Xylazine alone should not be used to sedate horses for hindlimb procedures including rectal palpation. Don’t use in horses on iv TMS (this drug formulation is rare in the US).
Occasional horses become xylazine aggressive and turn mean when sedated. Excessive or sudden stimulation can override the sedation.
Reversal : yohimbine (off the market); atipamezole
Contraindications: Be cautious in horses with heart disease. Do not use in neonates as it impairs cardiac output (cardiac output = heart rate x stroke volume; xylazine lowers heart rate but neonates can’t change their stroke volume to compensate). Use benzodiazepenes in neonates instead of alpha-2 agents.
Romifidine is an alpha-2 agonist that is similar to xylazine but with longer duration and less associated ataxia. It is popular with equine dentists.
Sedative dose: 0.04-0.12 mg/kg slow iv (10mg/ml)
Side effects and contraindications: Side effects are similar to xylazine with occasional episodes of colic after its use. Rarely a horse will become excited rather than sedated.
Detomidine is yet another alpha-2 agonist with even longer duration effects. Detomidine does a better job of balancing the horse’s weight (4 point stance) and is safe to use when working on the hindlimbs.
Detomidine is routinely given iv or im but does come as an oral gel that can be used for fractious horses (it does take 30 min to kick in). Typical doses are 0.5-1 ml (10 mg/ml).
Detomidine has a ceiling effect so increasing doses increase duration rather than increase sedation.
Side effects are the same as for xylazine but more significant due to the longer duration.
Acepromazine is a tranquilizer or calming agent. It does not have analgesic properties. It can be given orally, im or iv. It does have a delayed onset of action; 20 minutes may be required even with iv administration.
Dose iv or im: 2-4 ml (10mg/ml) for 1000# horse
Side effects: Ace is safe in most horses. Caution is needed in stallions and those horses in shock or with anemia. Ace lowers blood pressure and hematocrit. Ace can cause penile prolapse and priapism (persistent erection) in stallions and in geldings with urethral irritation. This condition can rapidly lead to fertility issues.
Diazepam and Midazolam
Schedule IV drugs.
Diazepam and midazolam are primarily used to relieve anxiety and enhance the sedative effects of other drugs. They act as sedatives in neonates.
Diazepam and midazolam are the preferred sedatives for neonates. Neonates cannot adjust stroke volume so have significant cardiovascular depression with the alpha-2 agents. (Cardiac output = SV * HR)
These benzodiazepenes are used interchangeably. Price variations are common so the cheapest drug is generally the one used. Diazepam must go iv, is light sensitive and cannot be stored in plastic (syringes). It generally cannot be mixed with other drugs (ketamine is okay). Midazolam can be given im and does not bind to plastic. It can be mixed with most other drugs and rectal administration may be effective.
Dose: 0.02-0.4 mg/kg
Side effects: both drugs are metabolized by the liver and the cytochrome P450 system. As such, they can be affected by and can affect the metabolism of other drugs.
The drugs are also used as anticonvulsants, muscle relaxants and appetite stimulants.
Anesthesia, Sedation, and Pain Management of Donkeys and Mules, 2019 VCNA Vol.35(3), pp.515-527
Anesthesia and analgesia for standing equine surgery, Vet Clin N Amer 2014
Plumb’s Veterinary Drug Handbook and app (lots of versions out there in different formats and prices)